This non-provisional application claims priority under 35 U.S.C. xc2xa7119(a) on patent application No. 2001-90799 filed in Japan on Mar. 27, 2001, which is herein incorporated by reference.
1. Field of the Invention
This invention relates to an exhaust emission control system of an internal combustion engine capable of post injection for raising the exhaust temperature.
2. Description of Related Art
An exhaust emission control device is known to reduce harmful matters in exhaust emitted from an internal combustion engine. For example, the exhaust emission control device applied to a diesel engine has a filter disposed in an exhaust passage, and has a function of collecting particulate matters included in exhaust. If the particulate matters have been deposited on the filter to a degree, the exhaust emission control device removes the particulate matters from the filter to recover the collecting capability of the filter. To recover the collecting capability of the filter, the filter is heated using an external heat source such as an electric heater to oxidize the particulate matters by the resulting heat, but it is most preferable to raise the temperature of the filter by means of heat of the exhaust from a standpoint of improvement in total fuel economy and reduction in size of the device as a whole.
Accordingly, a technique has been adopted in which an increasing amount of fuel is supplied according to the load of the internal combustion engine to raise the exhaust temperature by combustion heat of the fuel. In the case of a diesel engine, for example, fuel is injected into each cylinder in main injection and additional fuel is then injected in an expansion stroke (post injection) to raise the exhaust temperature by combustion heat of the fuel.
However, the conditions that realize the combustion as expected by the post injection are greatly influenced by atmosphere in cylinders unlike the self-ignition by main injection or the like. For example, if the cylinder temperature is too low, it is impossible to satisfactorily raise the exhaust temperature since the fuel cannot be completely combusted even if a large quantity of fuel is injected in the post injection. Further, if the post injection timing comes too early in terms of crank angle, a large amount of combustion heat is consumed by torque, resulting in the occurrence of a shock and making it impossible to raise the exhaust temperature as expected. Namely, the conventional exhaust emission control device of the internal combustion engine cannot achieve satisfactory post injection.
It is therefore an object of the present invention to provide an exhaust emission control system of an internal combustion engine, which realizes post injection with high accuracy to enable an efficient rise in the exhaust temperature.
To accomplish the above object, an exhaust emission control system of an internal combustion engine having a fuel injection device that carries out main injection of fuel into cylinders of the internal combustion engine and then carries out sub injection of additional fuel in an expansion stroke, comprising: an exhaust emission control device disposed in an exhaust passage of the internal combustion engine and is capable of reducing harmful matters in exhaust; an operating state detecting device that detects an operating state of the internal combustion engine; a target injection timing setting device that sets a target injection timing in the sub injection according to the operating state of the internal combustion engine; and a control device that provides control such that injection timing in the sub injection is set at a point earlier than the target injection timing when the sub injection is started, and the injection timing in the sub injection is then delayed to the target injection timing.
Originally, the target values of the fuel injection quantity and injection timing that enable a rise in the exhaust temperature in the most efficient manner in the sub injection can be set according to the operating state of an internal combustion engine such as the engine load and the engine speed.
On the other hand, a combustion heat resulting from the actual sub injection varies according to the conditions of atmosphere in cylinders. For this reason, if the temperature conditions that enable the efficient rise in the exhaust temperature have not yet been satisfied since the cylinder temperature is low, fuel cannot be combusted in an ideal manner even if the sub injection is started at the same injection timing as the target injection timing from the beginning. It is therefore impossible to raise the exhaust temperature as expected.
To address this problem, the exhaust emission control device according to the present invention provides control such that the actual sub injection is started in the injection timing earlier than the set target injection timing, and the injection timing in the sub injection is then delayed to the target injection timing in a predetermined period of time.
Therefore, the required temperature conditions are satisfied prior to the subinjection in the original target injection timing, and the sub injection realizes the efficient combustion and rise in the exhaust temperature at a point in time when the injection timing reaches the target injection timing. This enables an improvement in performance of the exhaust emission control device.
Further, a change in torque can be inhibited to a larger extent than in the case where the post injection is carried out in the target injection timing at a time, and it is therefore possible to prevent the drivability of a vehicle from deteriorating.